Electrosurgical apparatus which produces high frequency currents for cutting tissue and the coagulation of small blood vessels in electrosurgery are well known.
Recent examples of such apparatus are found in U.S. Pat. Nos. 4,569,345, 4,574,801 and 4,617,927.
Certain conditions of the return electrode may result in a burn to the patient. Some attempts have been made to monitor the return electrode to avoid injury to the patient. The disclosure of U.S. Pat. Nos. 4,416,276 and 4,416,277 to Newton et al. are directed to monitoring the return electrode to insure the return electrode is in place and to avoid injury to the patient.
The monitoring approach taken by Newton et al. is to use a voltage divider and a synchronous detector along with an automatic rather than a manual operation. Newton et al. automatically interprets a safe resistance representative of safe pad contact as the lowest resistance value observed over some period of time and automatically sets the upper limit of resistance as a function of that value. The disadvantage of this approach is that the circuit is more complex and therefore more costly and is less safe than a manual operation. An automatic operation does not allow the user which is typically the clinical staff to register concurrence that the pad has been properly applied. Moreover, this disclosure does not provide a display of the measured resistance.